LEAF AND SEEDLING DISEASES 175 
considered to be vestigial male cells (spermatia) though by 
some authors they are thought to be conidia or spores 
(pycnopores). They have never been known to germinate, 
and in any case they are now functionless. 
This very brief description of the life-history of a hete- 
roecious rust is only intended to assist those unacquainted 
with the group to follow the account of the two rusts which 
infect the larch. A more comprehensive account will be 
found in any botanical text-book or in Grove (1913). 
Melampsoridium betulinum, Kleb. (=Peridermium laricis). 
As the heteroecious rusts have frequently been known as 
different fungi on their two hosts, they have commonly been 
designated by two names. For instance, Peridermium laricis 
is an aecidial form which is found on larch needles. The 
uredo- and teleuto-form which occurs on the birch has also 
been called Melampsoridium betulinum (or formerly Melamp- 
sora betulina, Desm.). As it is cumbrous to keep two different 
names for one fungus, it has become the accepted practice to 
designate a rust whose complete life-history is known by the 
name applied to the uredo-form. Consequently thename Peri- 
dermium laricis, by which this rust is most generally known 
to foresters, has to give place to Melampsoridium betulinum. 
Fig. 72 shows the general appearance of this rust on the 
larch. The aecidia are very conspicuous by reason of the 
large white sheath which surrounds each pustule. This 
sheath, which is called a peridium, is one cell in thickness, 
and is shown enlarged in fig. 72, 4, and fig. 72, 7 shows the 
arrangement of the cells of the peridium when a portion of 
it is placed flat on a slide under the microscope. The 
Descrietion oF Fic. 72. 
1. Photograph of twig of Larix Europea with diseased leaves: (a) 
pseudoperidium with lacerate margin; (b) unopened pseudo- 
peridium ( x 4). ’ ao 
. Photogmph of twig of ditto, with fused pseudoperidia (x 4). 
Photograph of twigs of ditto (about natural size). 
A pseudoperidium showing lacerate margin (x about 50). 
Spore in surface view ( x 1000). 
. Spore in optical median section ( x 1000). . 
Part of pseudoperidium ; the lower cells show the verrucose marking 
on the walls ( x 220). 
(After A. W. Borthwick and Malcolm Wilson.) 
NOC ws 
